Many materials and devices such as leather and metal springs have long been employed to ameliorate the effect of impact shock and vibration on otherwise contiguous objects. The principal of shock absorption is to mitigate deleterious effects by distributing, both in time and space, the forces generated by shock impact and vibration. Negative effects of such forces on impacting objects include excessive wear, by frictional contact, deformation, destruction, etc. Primarily, a conventional shock absorbing article, such as an elastomeric pad, will disperse the forces over a broader surface area and extend the time over which the force is applied to the protected article. Although adequate for many purposes, such dispersal may well prove inadequate in particular situations.
For example, in the farriery art, the benefit of shock absorption from a horseshoe pad has long been recognized. In particular, placing a pad between a horseshoe and the hoof serves to reduce frictional contact as well as to mitigate the effect of shock on the horse's hoof and leg thereby lessening the need for veterinary care and painkilling drugs. Leather pads have long been used for this purpose. However, leather, when employed as horseshoe pads, is disadvantaged because it is not dimensionally stable under compressive stress and therefore loosens rather quickly. Furthermore, leather absorbs liquids which in the environment of a stall primarily means water and urine; a severe disadvantage.
To overcome the problems associated with the use of leather or cloth pads, synthetic polymeric pads were introduced to the farriery art. Wallen, in U.S. Pat. No. 3,747,684, defines certain of the shortcomings associated with leather, felt and rubber horseshoe pads. More specifically, Wallen, recites the lack of rigidity and wear resistance associated with pads made from these materials. He suggested shock absorbing pads composed of hard polyurethanes (Shore A Hardness 77-87). However, Wallen notes the heat instability of such pads and a five week storage requirement for permitting sufficient polymerization and, therefore, strength to his pads. McDonald in U.S. Pat. No. 3,603,402 discloses a high impact resistant polymer horseshoe pad composed of polycarbonates. U.S. Pat. No. 3,628,608 issued to Sherman, teaches an elastomeric horseshoe pad incorporating dispersed metal particles for shock absorption. Sherman recognizes the durability requirements of horseshoe pads and as a result includes the particles to enhance structural stability and wearability.
The conventional resilient materials used for construction of horseshoe pads discussed above, like conventional shock absorbers, merely redistribute shock impact force in both time and space. The use of rubbery pads presents an additional consideration; "springiness" exhibited upon shock impact. Such rebounding properties may actually amplify the effects of shock impact because of the tendency to generate an oppositely disposed reaction force. An ideal illustration of this type of behavior is the rebounding capacity of a "live" tennis ball in contrast to a "dead" ball. In the context of a shock absorbing pad, such rebounding is often undesirable.
It is desirable to incorporate shock absorption features in devices relating to other activities. In the case of athletics, trauma may be sustained as a result of impacts with hard surfaces; hence the development of a multitude of pads and protective coverings. More particularly, persons involved with the care of athletes have observed that tremendous impact forces are sustained by the legs, knees and the spinal cord from running. Repeated impacts associated with running can lead to deterioration of the skeletal and cartilaginous tissues. Attempts to solve this problem have led to the adoption of manufacturing practices and materials to minimize trauma. For example, shoe soles are now constructed of laminated resilient shock absorbing materials, each laminate having different time and space shock-redistribution characteristics, to enhance shock absorption. Additional protective features include wider shoe soles for distributing the impact shock across a larger area and longer time; and, therefore, reducing the pressure area loading and rate of application of force on the foot and the body.
Shock absorbing devices often find use in other environments involving a human interface associated with impact forces. The operation of pneumatic hammers and similar tools is based on impact forces. Accordingly, mere redistribution of shock forces from continuously repeated impacts, serves little protection for the operator or, for that matter, the non-impacting tool components.
The various devices discussed above, as well as others dedicated to shock absorption, share the ability to redistribute shock and vibration forces but fail to provide a means for actually damping the energy underlying the forces. None of the aforementioned padding devices possess the capacity for shock impact energy conversion to an alternate energy form.